CHAPTER VIII 

 THE TERTIARY ERA : THE EOCENE 



AGES must have passed while the ocean flowed over this 

 part of the world, and the chalk mud, with its varied 

 remains of living things, gradually accumulated at the 

 bottom. At last a change came. Slowly the sea bed 

 rose, till the chalk, now hardened by pressure, was raised 

 into land above the sea level. As soon as this happened, 

 sea waves and rain and rivers began to cut it down. 

 There is evidence here of a wide gap in the succession of 

 the strata. Higher chalk strata, which probably once 

 existed, have been washed away, while the underlying 

 strata have been planed off to an even surface more or 

 less oblique to the bedding-planes. The highest zone of 

 the chalk in the Island (that of Belemnitella macronata] 

 varies greatly in thickness, from 150 ft. at the eastern end 

 of the Island to 475 at the western. The latest investiga- 

 tions give reason to conclude that this is due to gentle 

 synclines and anticlines, which have been planed smooth 

 by the erosion which preceded the deposition of the next 

 strata, the Eocene.* At Alum Bay the eroded surface 

 of the chalk may be seen with rolled flints lying upon it, 

 and rounded hollows or pot-holes, the appearance being 

 that of a foreshore worn in a horizontal ledge of rock, 

 much like the Horse Ledge at Shanklin. 



The land sank again, but not to anything like the depth 

 of the great Chalk Sea. We now come to an era called 

 the Tertiary. The whole geological history is divided 



* See Memoir of Geological Survey of I. W. by H. J. Osborne 

 White, F.G.S. 1921, p. 90. 



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